1. Field of the Invention
The present invention relates to porphyrin compounds, processes for producing porphyrin compounds, and three-dimensional optical recording materials and three-dimensional optical recording media containing porphyrin compounds.
2. Description of the Related Art
Information recording media are used for various applications including business, home, and personal uses. Accordingly, information recording media are very important in modern industries and societies. Among the information recording media, currently, optical recording media on which information is recorded optically are used particularly widely. This is because the optical recording media have ease of information recording, a large recording capacity, etc. Examples of mainstream optical recording media include CDs, MOs, DVDs, etc. These optical recording media contain suitable optical recording materials and thereby allow information to be recorded thereon. In order to further increase the recording capacities of the optical recording media, research and development of better optical recording materials have been proceeding.
An example of the aforementioned optical recording materials is a material that allows information to be recorded by a recording method that is referred to as a “heat mode”. The heat mode is a method in which the change in refractive index is used. The change in refractive index is caused in the optical recording material by a thermal effect generated upon light irradiation. However, the heat mode has a limitation in recording capacity due to the diffraction limit of light. Hence, a study for recording or erasing information by a recording method that is referred to as a “photon mode” is being made. This photon mode allows a larger volume of information to be recorded as compared to the heat mode. For the photon mode, a photochromic material containing a photochromic compound (photochromic molecule) is used as an optical recording material. The photon mode allows information to be recorded by utilizing the isomerization of the photochromic compound that is caused by photon absorption.
The photochromic compound (photochromic molecule) is a compound (molecule) that exhibits photochromism. The photochromic material is an optical recording material that is formed of the photochromic compound and allows information to be recorded or erased by photochromism. The photochromism is a phenomenon in which a chemical species is isomerized reversibly between two states that are different in absorption spectrum, by light irradiation. Examples of the method of using the photochromic material include a method in which light irradiation is carried out repeatedly with the irradiation wavelength being varied. For example, this method makes it possible to repeat information recording and erasing by repeating bleaching and coloring that result from isomerization (photochromism) of the photochromic compound. Various compounds are being studied to be used practically as such a photochromic material. Among the photochromic compounds, recently, particularly organic compounds are being researched and developed actively.
Examples of currently known photochromic compounds include azobenzenes, diarylethenes, spiropyrans, fulgides, indigos, etc. However, with these conventional photochromic compounds, two-dimensional recording can be achieved but three-dimensional recording is difficult. This is because the photoisomerization reaction is caused by one-photon absorption in those conventional photochromic compounds. Accordingly, optical recording media such as CDs, MOs, DVDs, etc. that are used mainly at present are two-dimensional optical recording media (two-dimensional optical memory media). In the case of the two-dimensional optical recording media, information is recorded two-dimensionally, but the thickness direction of the media is not utilized. Thus, the thickness direction of the optical recording media cannot be utilized for information recording and this poses a limitation on recording capacity.
In order to solve this problem, construction of three-dimensional optical recording media (three-dimensional optical memory media) can be considered. In the three-dimensional optical recording media, information is written in the inner part thereof while being superposed in a plurality of layers. Accordingly, with the three-dimensional optical recording media, a considerable increase in recording capacity can be expected. In order to construct the three-dimensional optical recording media, the use of a photochromic compound that is isomerized by two-photon absorption is considered to be useful.
The two-photon absorption denotes that two photons in the wavelength range where one-photon absorption does not occur are absorbed simultaneously. Two-photon absorption makes it possible to generate an excitation state corresponding to the sum of energy of two photons. Accordingly, two-photon absorption can be caused selectively only at a position with a higher optical density such as a focal point in an optical recording material. Hence, two-photon absorption allows optical absorption to be controlled, with the three-dimensional position being selected in the optical recording material. It therefore is possible to carry out three-dimensional recording in which the thickness direction of the optical recording material also is utilized. Thus, when a photochromic compound (photochromic molecule) with a high two-photon absorption efficiency is developed and is used as an optical recording material, three-dimensional optical recording media can be produced.
Photochromic materials that utilize two-photon absorption also have been proposed in the past (see, for instance, D. A. Parthenopoulos, P. M. Rentzepis, Science 245 (1989) 843, and A. S. Dvornikov, Y. Liang, C. S. Cruse, P. M. Rentzepis, J. Phys. Chem. B 108 (2004) 8652). However, these photochromic materials are far from practical use since the two-photon absorption efficiency thereof is low (i.e. the two-photon absorption cross section is small).
The present inventors have invented a compound in which a porphyrin compound is linked using an ethynylene group (JP2004-168690A). This compound has a two-photon absorption efficiency that has been improved by two orders of magnitude or more as compared to those of conventional compounds. However, although this compound has a high two-photon absorption efficiency, the compound is difficult to undergo isomerization (photochromism) by optical absorption. Accordingly, the compound described in JP2004-168690A has a problem when being used as an optical recording material.